Kensington and Chelsea alone has seen 800 planning applications for basements in the five years to 2013.

But although it is reasonably easy to get a digger into the rear garden of a home, it can become almost impossible to get it out after it has become nestled at the bottom of a building site.

In the past, developers would have used a large crane to scoop up the digger – a costly and time-consuming process.

The new method, now considered standard operating practice, is to cover the digger with “hardcore”, a mixture of sand and gravel, with a layer of concrete poured over the top.

However, in some of the newest conversions of London's luxury homes, basement conversion specialists have encountered difficulties as they try to tuck “sub-basements” beneath the existing basement conversions.

They say they are increasingly coming across abandoned diggers from the last round of improvements.

A basement conversion can add anything up to 25 per cent on the price of a multi-million pound house.

Following the building of the Channel Tunnel, tunnel boring machines used on the English side were buried clear of the tunnel. On the French side, the machines were dismantled and removed.